I send them tenants that I suspect are going to be problem or otherwise undesirable. 

However, the same deal might be undesirable once I have 20% equity.

Or it is not an issue of morality in either case, but merely an inconvenient and undesirable (by some) phenomenon.

Clearly didn't leave on good terms, which is undesirable. :/ What would you do? 

Would you feel this is unsafe/undesirable to essentially have a stranger living on your property with you (short or long term)? 

More likely you have some seriously distressed D properties in undesirable neighborhoods, because the sale price has to include a profit margin for these guys, the original seller and the rehab crew.

Before I committed to buying the property, I knew it didn't fit the kind of property i wanted to own and it was what I call GUD (Geographically Un-Desirable).

Those points alone will filter out a lot of undesirable renters.

With plenty (90 days or more with an initial "head's up") of notice, I first "non-renewed", then moved out the undesirables, then made sure the vacant units were clean and functional, not doing any upgrades. 

Good points on sale-ability vs rent-ability of units in undesirable locations.